The present invention relates to an image forming apparatus for forming an image on a paper sheet.
In the image forming apparatus for forming an image on a paper sheet, in some cases a sheet is conveyed in a state where the sheet skews to a transfer position where an image is transferred to the sheet.
Ideally, the sheet is conveyed to the transfer position in a state in which the leading edge and the trailing edge of the sheet cross orthogonally the conveyance direction and the side edges are parallel to the conveyance direction, but the sheet is sometimes offset from this state and conveyed, and the leading edge reaches the transfer position in a state in which it is inclined with respect to the conveyance direction of the sheet. This phenomenon is called skewing or inclination, and improvements have been done in order to prevent this skewing.
The most widely used technique for preventing skewing is so-called “loop conveyance” using a registration roller.
“Loop conveyance” is a technique in which a sheet is conveyed by a loop forming roller, and the sheet is caused to abut the registration roller that has been stopped and conveyance of the sheet by the loop forming roller is continued and by forming a loop upstream of the registration roller, the leading edge of the sheet is caused to cross orthogonally the conveyance direction.
After the loop is formed upstream of the registration roller, conveyance starts and the sheet is conveyed to the conveyance position.
The skew is corrected by this type of loop conveyance, but there is some limit to the skew correction using the registration roller in that loop conveyance is not sufficient in the recent image forming apparatuses that require high accuracy at the image position on the sheet.
In Unexamined Japanese Patent Application Publication Nos. 06-263287 and 10-212055, it has been proposed that each of a plurality of conveyance members arranged in parallel so as to cross orthogonally the sheet conveyance direction, is controlled based on the results from the sheet skew detector.
In Unexamined Japanese Patent Application Publication Nos. 06-263287 and 10-212055, the skew is corrected for two conveyance members that are arranged so as to orthogonally cross the conveyance direction by performing control based on the detection results from the detector.
The techniques of Unexamined Japanese Patent Application Publication Nos. 06-263287 and 10-212055 are effective for sheet skew correction, but insufficient for high accuracy skew correction.
In recent times, use of electrophotographic type image forming apparatus has been expanding in the field of short-run printing.
Compared to conventional office applications, printing requires higher image position accuracy and more types of papers are printed and thus there is a tendency for skewing to occur.
For this reason, the conventional techniques in Unexamined Japanese Patent Application Publication No. 06-263287 have become insufficient for preventing skew.
As shown in FIG. 7, in order to evaluate the degree of skew, the proportion of the offset amount ΔY in the conveyance direction Y due to a skew of the angle PA of the sheet with respect to the length PX in the direction X which orthogonally crosses the conveyance direction Y of the sheet P, or in other words the skew ratio is (ΔY/PX)×100%.
The prior art technology is effective for correcting an offset amount of about 1%, but keeping the permissible amount of offset required by recent image forming apparatuses to 0.2% or less is difficult.